Derivatives - Derivatives Associated with Fixed Income Trading Activities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Customer interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Notional	$ 2,200,080	$ 2,026,753
Assets	7,861	22,097
Liabilities	55,392	18,323
Offsetting upstream interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Notional	2,200,080	2,026,753
Assets	7,904	17,931
Liabilities	8,244	20,720
Option contracts purchased | Long
Derivative Instruments, Gain (Loss) [Line Items]
Notional	30,000	20,000
Assets	32	15
Liabilities	0	0
Forwards and futures purchased
Derivative Instruments, Gain (Loss) [Line Items]
Notional	5,425,631	6,257,140
Assets	4,915	4,354
Liabilities	14,002	5,526
Forwards and futures sold
Derivative Instruments, Gain (Loss) [Line Items]
Notional	5,701,595	6,292,012
Assets	15,098	5,806
Liabilities	$ 4,768	$ 4,010